In the competitive world of dieting, one of three Horsham-based weight-loss companies has taken an abrupt, mysterious fall.

The demise of Pure Weight Loss Inc. follows a split three years ago between two key business partners and a name change last summer. Pure Weight Loss was once part of LA Weight Loss Centers Inc., which still operates from the same office park.

In a terse statement, Pure Weight Loss said it would close Jan. 4 after 18 years because of "fierce competition from the Internet; the introduction of Alli, the over-the-counter weight-loss pill, and other commercial diet programs; combined with a difficult economic environment."

Pure Weight Loss, LA Weight Loss and NutriSystem Inc. all have headquarters in Horsham. LA Weight Loss is still in business, as is the publicly held NutriSystem.

An outside public relations firm hired to represent Pure Weight Loss said Vahan Karian, chairman and chief executive officer of the company, would not speak to the media. A call to the company's attorney was not returned. An employee of the public relations firm said she could not say how many diet centers and employees were affected by the closure.

In a separate statement, the company told customers to pick up by Jan. 4 any products they have ordered.

In August, when Pure Weight Loss announced the name change, it said it had 400 centers nationwide and described itself as "the country's largest weight-loss chain that offers both one-on-one counseling and the option to eat your own food."

Harold Katz, who started NutriSystem in the '70s and is also a former 76ers owner, founded LA Weight Loss 18 years ago. Ten years ago, he became partners with Karian, said Greg Matusky, an LA Weight Loss spokesman. Because of "differences of opinions and perspectives" on how to operate the company, the two split in September 2004. Karian kept control of what were known as corporate centers, most of which are in cities, including Philadelphia. Katz kept the franchising arm, which has 400 centers that tend to be in more rural areas. Their separate companies both used the LA Weight Loss name.

In July, Oregon's attorney general filed a complaint against LA Weight Loss Franchise Co., alleging "false and misleading representations about the costs, fees, products and benefits associated with the program." That case is ongoing.

Matusky said Katz learned of Pure's closing only a day before the official announcement: "We really knew nothing about their closing, and there was no sharing of information between the two companies in that four-year period."

The closing will make it possible for LA Weight Loss centers to operate in markets that previously were Pure's territory. "It's a competitive market," Matusky said, "and when one competitor goes out of business, it realigns the industry."

Michael Hagan, chairman and CEO of NutriSystem, said the closing of Pure Weight Loss would not affect his company, which uses a different, Internet-based business model. Hagan is an investor in Philadelphia Media Holdings L.L.C., which owns The Inquirer, the Philadelphia Daily News and Philly.com. "This is a big category," he said, "and I don't think our fortunes will rise or fall based on having Pure Weight Loss in the market or out of the market."

Contact staff writer Stacey Burling at 215-854-4944 or sburling@phillynews.com.